Gaseous conduction device



April 20, 1937. c. 6. SMITH GASECUS CONDUCTION DEVICE Original Filed May24. 1926 I/VERT 6A5 Patented Apr. 20, 1937 UNITED STATES GASEOUSCONDUGTION DEVICE Charles G. Smith, Medford, Mass, assignor, by

mesne assignments, toRaytheon Manufacturing Company, Newton, Mass, acorporation of Delaware Application May 24,

1926, Serial No. 111,278

Renewed April 18, 1936 9 Claims.

This invention relates to devices of the type having a tube containingelectrodes separated by a gaseousmedium, preferably at a low pressure,and particularly to gaseous conduction rectifiers, objects of theinvention being to produce a device of this type having "low totalvoltage loss, little if any cathode disintegration, small reversecurrent when used as a rectifier, little if any gaseous clean-up,protection against deleterious effects of 10 ions and radiation upon theinsulating material embodied in the tube, low starting voltage,stability of operation, and generally to improve devices of theaforesaid type.

In one aspect the invention comprises a gaseous conduction tube havingan anode, a cathode, and a cathode coating of caesium or other alkalimetal or alkali earth metal or other material characterized by a lowwork function, and either one or both of the characteristics, viz., alow vaporiz- 0 ing temperature, and being highly electropositive, thecathode being constructed and arranged to operate at a temperature belowthat at which the coating is driven 011'. Thus, when used as arectifier, for example, the tube permits current to flow freely when thecathode is negative while permitting practically noreverse current whenthe anode is'negative. While some of the material may be driven off thecathode, particularly when operating under conditions which cause thedis- 3 charge to are from a hot-spot, the cathode is sufiiciently largeto permit the discharge continu ously to shift to coated areas. Thecathode coating serves to facilitate the gaseous conduction and to lowerthe potential drop in the region of the 35 cathode, thereby lowering thetotal potential drop between cathode and anode.

By making the cathode large it can be maintained at an averagetemperature well below that at which the coating is rendered ineffectiveand by making the cathode hollow the coating material which is drivenoff may redeposit upon other areas so that even in the case of awandering are each spot from which the coating is driven off is soonrecoated by material driven off other spots.

In some cases, as when using caesium, the oathode coating may be verythin, even to'the extent of being invisible. Indeed, for certainpurposes satisfactory results may be obtained with the coating in theform of a layer of the vapor of the material adsorbed on the surface ofthe cathode, the vapor not only increasing the electron emission whendeposited in or on the surface of the cathode but also affording acopious supply of ions in the gap between cathode and anode by virtue ofits low ionization voltage. Of the vapor materials above referred tocaesium has the advantage of a low vaporization temperature, this metalhaving an appreciable vapor pressure even at room temperature.

60 The cathode coating adheres better and is more effective if thecathode surface is rough. An effective surface is formed by making thecathode of carbon or depositing a layer of carbon on a metal cathodebefore applying the caesium or other coating. Carbon is moreelectronegative than most surfaces and therefore serves better to bindthe caesium or other electropositive material to its surface. Thecathode, particularly when formed of metal instead of carbon, may becoated with an oxide, for example, an oxide of an alkali earth metal, orwith chromium, before applying the electropositive material such ascaesium. Or, the cathode, if formed of metal, may merelybe oxidized tomake its surface highly electronegative.

In another aspect the invention comprises the use of an inert gas withcaesium or other'of the aforesaid materials which have vapor pressurestoo low, at normal tube temperatures, alone to maintain gaseousconduction. The inert gas serves for starting purposes and when ionizedby the discharge, the developed heat evaporizes the caesium or the like,the vapor of Which is then ionized, affording a very low voltage drop.This combination of gas and vapor is particularly useful in gaseousrectifiers since the vapor alone, at temperatures high enough to producesufficient vapor pressure to maintain conduction, would tend to permitcurrent flow in both directions between cathode and anode.

I have found that vapors such as caesium tend to react with thematerials of the tube, particularly the glass container and otherinsulation parts, when ionized by radiation of ionized inert gas orotherwise, and I therefore prefer to shield such parts by confining thedischarge. The best way of accomplishing this shielding, so far as I amnow aware, is to make the cathode hollow and to present the anode to theinterior surface of the cathode so that most if not all of the radiationis confined to the interior of the cathode. The anode may extend througha restricted opening in the cathode with means adjacent the opening toobstruct the small amount of radiation which would otherwise emanatethrough the opening, and either one or both of the spacings between theanode and cathode opening and between one or both electrodes and theobstructing means may be restricted substantially to the mean free pathof electrons in the gas for insulation purposes. By thus confining theradiation it assists in maintaining a higher degree of ionization,thereby lowering the voltage drop between cathode and anode. Stillanother advantage in making the cathode .hollow consists in that thecoating material which is vaporized or otherwise dislodged from any areaof the cathode is redeposited upon other areas of the interior surfaceof the hollow cathode, thereby conserving the coating material andrestricting its escape to other parts of the tube where its effect isdeleterious.

Owing to the fact that a highly electropositive material such as caesiumreadily combines with moisture and other constituents of the atmosphere, it should not be introduced into the tube until the tube has beenevacuated or until the air has been replaced with inert gas.Consequently, I propose to incorporate, within a confined spaceincluding the interior of the tube such as an integral appendix,ingredients which react to produce the material, and then after the tubeis evacuated cause the ingredients to react, by application of heat orotherwise, to produce the material in the form of a vapor which depositsupon the surfaces inside the tube. To produce caesium I preferablyemploy calcium and caesium chloride. By placing the ingredients in ametallic cup or capsule they may be heated by current induced inthecapsule from a high frequency field generated outside the tube.

For the purpose of illustrating the invention characterized as aboveoutlined, one concrete embodiment is shown in the accompanying drawingin which,

Fig. 1 is an axial section of a tube;

Fig. 2 is a section on line 22 of Fig. 1;

Fig. 3 is a section on line 33 of Fig. 1;

Fig. 4 is a side elevation of the tube shown in Fig. 1 before it iscompleted;

Fig. 5 is a side elevation of a cathode-anode assembly as viewed fromthe left of Fig. 1;

Fig. 6 is a detail view of the filling capsule; and

Fig. 7 is a detail view of a modification.

The particular embodiment chosen for the purpose of illustrationcomprises a tube l of lime glass or other suitable material, a cathode2, two anodes 3, leads 2 to the anodes respectively, a lead 5 to thecathode, a shield 6, a support I for the shield, and a support 8 for thecathode.

The cathode comprises a cylindrical wall 9 and ends ill and II. One ofthe ends may be formed integrally with the cylindrical wall, but theyare both preferably formed separately and mounted over the ends of thewall. The ends may have cylindrical flanges which fit over thecylindrical wall as shown at l0 and H in Fig. 5 or they may be in theform of disks as shown in Fig. 1. In either case they are preferablywelded to the cylindrical wall, either continuously around the peripheryor at circumferentially spaced spots. The lower end H has two openingswith exterior flanges surrounding the reduced ends of the anodes inspaced relation thereto, the outer diameter of the flanges preferablybeing approximately equal to the diameter of the larger portions of theanodes. Mounted within the cathode are rings or washers i2 havingintegral fianges to hold them in spaced relation, the spaces between thewashers being very small to provide crevices from which the electronicdischarge emanates more freely than from a fiat surface. In theillustrative embodiment the cathode is approximately one inch indiameter, thus indicating the order of spacing between the washers l2.The washers may be mounted in the cathode in any suitable manner, as forexample, merely by a tight fit within the cylindrical portion of thecathode.

As explained above, the interior surface of the cathode is preferablyrough and is preferably coated with caesium or other material having alow work function. The rough surface may be provided merely by oxidizingthe surface of the cathode or by first coating it with carbon or with alayer of oxide. Such a surface will adsorb more of the caesium or otherelectropositive vapor than will a smooth surface. The cathode may beformed of nickel, iron, or more refractory metal such as tungsten ormolybdenum. When the cathode is formed of metal, its surface may be madehighly electronegative by oxidizing it. The cathode may also be formedof carbon, in which case it naturally has a rough surface without beingcoated on the inside although even in that case it may have a coating ofoxide.

The anodes may be formed of nickel, iron, or more refractory metal, andas illustrated in Fig. l the lead-in wires i extend through axialopenings in the ends and are secured to the anodes at the upper ends bybeing pinched in the tips or" the anodes. As shown in Fig. l the anodesrest upon hollow protuberances upon the re-entrant stem l3 of the tubei.

The shield 6 is preferably formed from a single piece of sheet metal asshown in Fig. 3. The space between the shield and each anode as well asbetween the shield and each flange of the cathode is preferably confinedapproximately to the mean free path of electrons in the gas, forinsulation purposes. The space between each cathode flange and eachanode is also preferably of the same order for the same purpose. Theshield 6 not only serves for insulation purposes but also obstructsradiation from the interior of the cathode to any part of the tube 5,including the junction between the tube and the anode wheredisintegration would otherwise occur by interaction between the caesiumand the tube material.

The air within the tube is preferably replaced with an inert gas such asneon. For low voltage tubes the neon pressure may be of the order of 5mm. while for higher voltage tubes the pressure should be somewhatlower. The deposit of caesium or the like above referred to is indicatedin Fig. 1 on the inside of the tube and on the inside of the cathode bystippling.

One method of filling the tube is illustrated in Fig. 4 wherein i iindicates a pump connection, connection to a source of inert gas and iiian integral appendix on the tube l. Mounted within the appendix i6 is ametallic capsule H contain ing ingredients which react to produce themetallic vapor such as caesium. As shown in Fig. 6 this capsule maycontain calcium and caesium chloride, the calcium being somewhat inexcess of the amount necessary to react with all of the caesium chiorideto produce caesium. In order to hold the two ingredients in separatedjuxtaposition, thereby to prevent their interaction except at very hightemperature, they are preferably separated by a partition which permitsthe ingredients to intermingle oniy at high temperature. As shown inFig. 6 the partition is formed as a wad of steel wool through which theingredients may pass when melted or vaporized. A similar wad ispreferably placed at the mouth of the capsule normally to hold thematerials in place.

During the pumping operation the inert gas is shut off and the tube isheated to drive off impurities. Current may be passed between thecathode and anodes further to assist in driving off the impurities.After the tube is thoroughly evacuated, the inert gas and caesium areadmitted. While the inert gas may be admitted first, it is preferablefirst to distill the caesium into the tube inasmuch as it passes in morefreely at the lower pressure within the tube. After both the inert gasand the caesium or the like have been admitted into the tube, theconnections are sealed ofi at I8 and I9 as indicated in Fig. 1.

While the ingredients in the capsule I! may be caused to react invarious Ways, the best way'of which I am aware is to heat theingredients by current induced in the metallic capsule I! from a highfrequency source such as indicated at outside the tube. Inasmuch ascaesium chloride melts at approximately 647 C. and calcium at about 810C., the latter subliming readily at 725 C., the whole tube or at leastthe capsule I1, is preferably heated only to approximately 600 C. todrive on impurities during the pumping operation, and subsequently'toapproximately at 850 C. to cause the ingredients to react to give oficaesium vapor. When the capsule I1 is mounted in an appendix as shown inFig.4, most of the caesium vapor condenses in the bottom of the tube asindicated at 2|, only a small part passing up into the body of the tube,in which case the deposit at 21 may be redistilled throughout theInstead of mounting the capsule H in an appendix l6 as illustrated inFig. 4, it may be'mounted in the body of the tube, as for example on thecathode 9 as illustrated at [1' in Fig. 5, in which case the caesiumvapor penetrates to the interior of the cathode through the restrictedopenings in the lower end or" the cathode more readily. In this case thecapsule isnot removed. 7

As shown in Fig. '7 the capsule I1" is mounted within the cathode andmay be heated either by current induced in the cathode by high frequencycurrent around the tube or by passing a discharge between cathode andanodes. This method has the advantage of depositingmost of the caesiumor the like upon the interior of the cathode; and

'by heating the cathode hot enough the coating material may be causedto; combine chemically with the cathode, particularly if the latter isformed of nickel. Instead of using calcium, magnesium may be used; .andinstead of caesium, potassium or the like may be used.

The particular embodiment of the invention shown in the drawing anddescribed in detail is included merely to illustrate the invention, myintention being not to limit myself to this embodiment but only to thegeneric invention defined by the appended claims.

I claim:

1. The method of introducing caesium into a space discharge tube whichcomprises incorporating within the tube calcium and caesium chlorideseparated from each other, removing the air from the tube, and thencausing said calcium and chloride to intermingle under raisedtemperature and to interact to produce caesium.

2. A vacuum tube comprising a plurality of electrodes, a capsule withinsaid tube containing ingredients which react to produce a differentsubstance, the capsule having an opening communicating with the interiorof the tube, and means separating said ingredients when cold butpermitting the ingredients to intermingle when heated.

3. A metallic capsule containing caesium chloride and calcium, and a wadof steel wool separating the two ingredients.

4. The method of introducing a dischargepromoting substance into a spacecurrent tube, which comprises incorporating within the tube a pluralityof reacting ingredients separated from each other, evacuating the tubewhile maintaining said ingredients separate, and subsequently heatingsaid ingredients and causing them to intermingle and react so as toproduce said substance.

5. The method of introducing an alkali metal into a space current tube,which comprises incorporating within the tube a reducible compound ofalkali metal and a reducing material separate from each other,subsequently evacuating said tube, and then passing said alkali metalcompound over said reducing material in heated condition to producereaction therebetween and supply the alkali metal.

6. A unidirectional gaseous discharge device comprising a hermeticallyclosed envelope containing an anode of small effective area and anon-thermionic cathode of nickel having a relatively large efiectivearea, a gaseous filling in said envelope, the surface of said cathodehaving formed thereon a layer of nickel oxide, and

stricting the discharge to' one end of said anode comprising a metallicsleeve surrounding said anode and its lead-in wire and extending from apoint adjacent the sealing point of said lead-in wire to a pointadjacent the end of said anode, said metallic sleeve being insulatedfrom said anode and lead-in wire andbeing spaced therefrom by a gap tooshort to permit ionization of the gas under the potentials applied.

8. A gaseous discharge device comprising a gas-tight vessel containing agas, a cathode and an anode, lead-in wires for said electrodes sealedthrough the wall of said vessel, means for restricting the discharge toone end of said anode comprising a metallic sleeve surrounding saidanode and its lead-in wire and extending from a from by a gap too shortto permit ionization of the gas under the potentials applied, saidmetallic sleeve also being insulated from said cathode.

9. A gaseous discharge device comprising a gastight vessel containing agas, a cathode and a plurality of anodes, lead-in wires for saidelectrodes sealed through the wall of said vessel, means for restrictingthe discharge to one end of each of said anodes comprising a metallicsleeve surrounding each of said anodes and their lead-in Wires andextending from a point adjacent the sealing point of said lead-in wireto a point adjacent the end of said anode, said metal lic sleeve beinginsulated from said anode and lead-in wire and being spaced therefrom bya gap too short to permit ionization of the gas under the potentialsapplied.

' CHARLES G. SMITH.

